Add a link to the crates.io security page in SECURITY.md

[carols10cents]

Crates.io now has its own security page that has information relevant to the broader Rust ecosystem. People may look for this information in rust-lang's SECURITY.md. I think it makes sense to have that information linked here just in case.

Pietro is working on linking to https://crates.io/policies/security from https://www.rust-lang.org/policies/security, the latter of which is already linked here, but that's 2 clicks away.

I am open to wording changes to make this clearer; I am also open to this being rejected as not relevant in this location or not needed because the Rust security page will soon link to the crates.io security page.

But I'm on a mission to spread info that the Foundation can help crate authors with security problems by getting the information out as much as possible, so I figured I'd try adding it here and see what folks think.

[Manishearth]

I'm in favor, speaking for myself. cc @pietroalbini @cuviper

[pietroalbini]

Hmm, I'm not sure if this is the place for a link to the crates.io security policy to go. This would be shown in every repository's home page (example), and the link is not relevant for any repository except for maybe crates.io itself.

I'll try to prioritize getting the scope section merged into the security policy though, to hopefully alleviate the issue.